The use of Injection Locked Oscillators had received widespread attention in communication circuits and in frequency synthesis applications. For example, its advantages with respect to locking speed and selectivity is covered in articles by Adler, R. entitled `A Study of Locking Phenomena in Oscillators.`, Proceedings of the I.R.E, vol. 34; pp. 351-357, (1946)--hereinafter refered to as the Adler paper; by Uzunoglu,V and White, M. H. entitled `The Synchronous Oscillator: A Synchronization and Tracking Network.`, IEEE Journal of Solid-State Circuits, vol. SC-20; (6), pp. 1214-1225, (1985); by Armand, M. entitled `On the Output Spectrum of Unlocked Driven Oscillators.`, Proceedings of the IEEE (Letters), vol. 57; pp. 798-799,(1969)--hereinafter refered to as the Armand paper
The ILO locks on a signal source if the source is within the frequency lock range of the ILO. Under this lock condition, the ILO generates a signal with a frequency that is identical to that of the driving signal with a transfer phase shift and it provides effective filtering of lower level spurious frequency components that might be present at the driving input to the ILO. The early work in Adler's ILO paper provides the transfer phase relationship and the lock range as a function of the ILO resonant circuit Q and the input drive level. In the lock range the output amplitude is fixed and equal to oscillator level wherein the output phase with respect to the nominal phase at the center of the lock range is given by an arcsine function: .theta.=arcsin(.function./a), wherein .function. is the frequency offset from the center free running
frequency of the ILO and a is the frequency lock range which is proportional to the drive level amplitude.
The transfer characteristics outside the lock range can be obtained from the first term of the Fourier analysis given in Armand's paper for the spectrum of an unlocked driven oscillator.
Following the Armand paper we have for .vertline..function..vertline.&gt;a the out-of-lock response as a function of the frequency offset with respect to the free running ILO frequency is proportional to ##EQU1##
Therefore, in our model, the composite ILO response is given by Equation (2). ##EQU2##
The term a.multidot.V.sub.a is due to drive level leakage into the ILO output structure and is proportional to input level through a and the complex coupling coefficient V.sub.a, which typically is in the order of -0.1 (-20 dB and out-of phase). This composite response is shown in FIG. 1 for such typical V.sub.a value and in FIG. 2 for V.sub.a of the same magnitude but with a phase of -45.degree.. As noted above FIG. 1 shows the Injection locked oscillator (ILO) selectivity characteristics. This phase-magnitude transfer function represent the response of the ILO to a constant level drive signal and is a finction of the normalized frequency centered about the free running frequency of the ILO. FIG. 2 shows that ILO response can often be skewed by phase offset of the driver leakage term. Here, the phase coupling is at -45 degrees. Alternatively, phase coupling of -135 degrees will produce an opposite effect of excentuating lower frequencies at the expense of higher frequencies.
The value of the ILO in signal synthesis could be realized only if its center frequency can be maintained so that it is within the lock range from the frequency of a desired signal source that one wishes to reproduce with significant rejection of undesired spectral components. Prior art methods of achieving that goal have been described by Joynson et al. In U.S. Pat. No. 5,107,272 issued on Apr. 21, 1992. By utilized the special phase transfer relationship, Joynson et al vary the drive frequency to track the free running frequency of the ILO so as to maintain lock. However, it is often desired to use a synthesized drive signal of a predetermined and non-variable frequency and still be able to reliably maintain ILO lock without unduly broadening the lock range. This will permit the accommodation of thermal drifts while still maintaining a superior selectivity due to lower lock range.